Many applications require a low-cost, low-power electric motor. Exemplary applications include small appliances, such as dishwashers, and pumps for use in spas and pools. In such applications, it is common to use the following motor types: permanent magnet (“PM”) synchronous electric motor, three-phase brushless permanent magnet (BPM) motors, and induction motors.
These motor types suffer from various disadvantages. For example, a PM synchronous electric motor is limited to commutation at the same frequency as the AC line-in (e.g. 60 Hz in the US and 50 Hz in Europe). This causes audible noise because the human ear is sensitive to vibrations at or near these frequencies (and/or harmonics of these frequencies). Moreover, a PM synchronous motor is generally limited to a single speed.
A two-phase or three-phase BPM motor may produce less audible noise than a PM synchronous motor, and may be operated at variable speeds. However, disadvantages of two-phase and three-phase BPM motors include higher cost and complexity.
Advantages of single-phase BPM motors include lower cost and reduced complexity relative to two-phase or three-phase BPM motors. However, the industry has avoided the use of a single-phase BPM motors in many applications (including dishwashers) due to two primary problems: (1) noise related to the fundamental frequency (e.g. 50 Hz or 60 Hz), and (2) noise caused by torque ripple. The inventors herein have developed innovative techniques for overcoming each of these problems in single-phase BPM motors.
Exemplary embodiments disclosed herein include an innovative single-phase electric motor that can be electronically commutated at frequencies other than the AC line-in frequency. For example, an exemplary system disclosed herein includes a single-phase BPM electric motor that is electronically commutated at 50 Hz or less. In an exemplary embodiment the motor is electronically commutated at 38 Hz. One advantage of this innovative technique is a reduction in human-audible noise levels during motor operation.
Exemplary embodiments disclosed herein include innovative techniques for shaping the drive waveform for powering an electric motor to approximate the counter-electromotive force of the electric motor. Advantages of this wave shaping technique include improved motor efficiency resulting in higher torque at a given electric power level (torque per amp), and reduction in human-audible noise levels. Advantages of this wave shaping technique also include reduction of vibration due to torque ripple.